Non-invasive brain perfusion MRI using endogenous deoxyhemoglobin as a contrast agent: preliminary data

BACKGROUND: The paramagnetic properties of deoxyhemoglobin shorten T2* as do gadolinium based contrast agents. Induction of abrupt changes in arterial deoxyhemoglobin concentration ([dOHb]) can mimic the action of intra-vascular boluses of gadolinium based contrast agents (GBCAs) used for perfusion imaging. AIM: To demonstrate the feasibility of rapidly changing pulmonary venous hemoglobin saturation for generating boluses of blood with altered T2* properties for measuring flow metrics in the systemic circulation. METHODS: A gas blender with a sequential gas delivery breathing circuit and software enabling prospective arterial blood gas targeting was used to implement rapid lung changes in the partial pressure of blood oxygen (PaO2) while maintaining isocapnea. Lung PaO2 was initially lowered to induce a low baseline deoxyhemoglobin concentration [dOHb]. PaO2 was then rapidly raised to normal for 10 seconds and then rapidly lowered to the initial low baseline creating a oxyhemoglobin (OHb) bolus. R2* changes were measured using blood oxygenation dependent (BOLD) MRI signal changes in large arteries and veins as well as in the microcirculation. This enabled generation of the following maps: bolus arrival time delay (TD) cerebral blood volume (CBV), mean transit time (MTT) and cerebral blood flow (CBF). RESULTS: BOLD signal in the middle cerebral artery showed a sharp increase during the OHb bolus transit indicating minimal dispersion confirming effective rapid modulation of pulmonary venous PO2 with reasonable cortical contrast-to-noise ratio of 3. Signals sorted by amplitude of signal changes and arrival times enabled the visualization of major arteries and veins. Contrast to noise ratio was adequate for a single gas challenge to provide most of the contrast, little improved by averaging over the remaining set of challenges. Values of the flow metrics derived from the perfusion maps were within normal ranges from published literature values. CONCLUSION: Non-invasive induction of abrupt changes in OHb saturation can function as a novel non-invasive vascular contrast agent for use in perfusion imaging.